1. Field of the Invention
This invention involves the preparation of anionic clays.
2. Prior Art
Anionic clays have a crystal structure which consists of positively charged layers built up of specific combinations of metal hydroxides between which there are anions and water molecules. Hydrotalcite is an example of a naturally occurring anionic clay, in which carbonate is the predominant anion present. Meixnerite is an anionic clay wherein OHxe2x88x92 is the predominant anion present.
In hydrotalcite-like anionic clays the brucite-like main layers are built up of octahedra alternating with interlayers in which water molecules and anions, more particularly carbonate ions, are distributed. The interlayers contain anions such as NO3xe2x88x92, OH, Clxe2x88x92, Brxe2x88x92, Ixe2x88x92, SO42xe2x88x92, SiO32xe2x88x92, CrO42xe2x88x92, BO32xe2x88x92, MnO4xe2x88x92, HGaO32xe2x88x92, HVO42xe2x88x92, ClO4xe2x88x92, BO32xe2x88x92, pillaring anions such as V10O28xe2x88x926 and MO7O246xe2x88x92, monocarboxylates such as acetate, dicarboxylates such as oxalate, alkyl sulfates such as laurylsulfonate.
It should be noted that a variety of terms are used to describe the material, which is referred to in this patent as an anionic clay. Hydrotalcite-like and layered double hydroxide are interchangeably used by those skilled in the art. In this patent application we refer to the materials as anionic clays, comprising within that term hydrotalcite-like and layered double hydroxide materials.
The preparation of anionic clays has been described in many prior art publications. Recently, two major reviews of anionic clay chemistry were published in which the synthesis methods available for anionic clay synthesis have been summarized, F. Cavani et al xe2x80x9cHydrotalcite-type anionic clays: Preparation, Properties and Applications, Catalysis Todayxe2x80x9d, 11 (1991) Elsevier Science Publishers B. V. Amsterdam. J P Besse and others xe2x80x9cAnionic clays: trends in pillary chemistry, its synthesis and microporous solidsxe2x80x9d (1992), 2, 108, editors: M. I. Occelli, H. E. Robson, Van Nostrand Reinhold, N.Y.
In these reviews the authors state that a characteristic of anionic clays is that mild calcination at 500xc2x0 C. results in the formation of a disordered MgO-like product. Said disordered MgO-like product is distinguishable from spinel (which results upon severe calcination) and from anionic clays. In this patent application we refer to said disordered MgO-like materials as Mgxe2x80x94Al solid solutions. Furthermore, these Mgxe2x80x94Al solid solutions contain a well-known memory effect whereby the exposure to water of such calcined materials results in the reformation of the anionic clay structure.
For work on anionic clays, reference is given to the following articles:
Helv. Chim. Acta, 25,106-137 and 555-569 (1942)
J. Am. Ceram. Soc., 42, no. 3, 121 (1959)
Chemistry Letters (Japan), 843 (1973)
Clays and Clay Minerals, 23, 369 (1975)
Clays and Clay Minerals, 28, 50 (1980)
Clays and Clay Minerals, 34, 507 (1996)
Materials Chemistry and Physics, 14, 569 (1986).
In addition there is an extensive amount of patent literature on the use of anionic clays and processes for their preparation.
European Patent Application 0 536 979 describes a method for introducing pH-dependent anions into the clay. The clay is prepared by the addition of a solution of Al(NO3)3 and Mg(NO3)2 to a basic solution containing borate anions. The product is then filtered, washed repeatedly with water, and dried overnight. Additionally mixtures of Zn/Mg are used.
In U.S. Pat. No. 3,796,792 by Miyata entitled xe2x80x9cComposite Metal Hydroxidesxe2x80x9d a range of materials is prepared into which an extensive range of cations is incorporated, including Sc, La, Th, In, etc. In the examples given solutions of the divalent and trivalent cations are prepared and mixed with base to cause co-precipitation. The resulting products are filtered, washed with water, and dried at 80xc2x0 C. Example 1 refers to Mg and Al and Example 2 to Mg and Bi. Other examples are given, and in each case soluble salts are used to make solutions prior to precipitation of the anionic clay at high pH.
In U.S. Pat. No. 3,879,523 by Miyata entitled xe2x80x9cComposite Metal Hydroxidesxe2x80x9d also a large number of preparation examples is outlined. The underlying chemistry, however, is again based on the co-precipitation of soluble salts followed by washing and drying. It is important to emphasize that washing is a necessary part of such preparations, because to create a basic environment for co-precipitation of the metal ions a basic solution is needed and this is provided by NaOH/Na2CO3 solutions. Residual sodium, for example, can have a significant deleterious effect on the subsequent performance of the product as a catalyst or oxide support.
In U.S. Pat. No. 3,879,525 (Miyata) very similar procedures are again described.
In U.S. Pat. No. 4,351,814 to Miyata et al. a method for making fibrous hydrotalcites is described. Such materials differ in structure from the normal plate-like morphology. The synthesis again involves soluble salts. For example, an aqueous solution of a mixture of MgCl2 and CaCl2 is prepared and suitably aged. From this a needle-like product Mg2(OH)3Cl.4H2O precipitates. A separate solution of sodium aluminate is then reacted in an autoclave with the solid Mg2(OH)3Cl.4H2O and the product is again filtered, washed with water, and dried.
In U.S. Pat. No. 4,458,026 to Reichle, in which heat-treated anionic clays are described as catalysts for aldol condensation reactions, again use is made of magnesium and aluminum nitrate salt solutions. Such solutions are added to a second solution of NaOH and Na2CO3. After precipitation the slurry is filtered and washed twice with distilled water before drying at 125xc2x0 C.
In U.S. Pat. No. 4,656,156 to Misra the preparation of a novel absorbent based on mixing activated alumina and hydrotalcite is described. The hydrotalcite is made by reacting activated MgO (prepared by activating a magnesium compound such as magnesium carbonate or magnesium hydroxide) with aqueous solutions containing aluminate, carbonate and hydroxyl ions. As an example the solution is made from NaOH, Na2CO3 and Al2O3. In particular, the synthesis involves the use of industrial Bayer liquor as the source of Al. The resulting products are washed and filtered before drying at 105xc2x0 C.
In U.S. Pat. No. 4,904,457 to Misra a method is described for producing hydrotalcites in high yield by reacting activated magnesia with an aqueous solution containing aluminate, carbonate, and hydroxyl ions.
The methodology is repeated in U.S. Pat. No. 4,656,156.
In U.S. Pat. No. 5,507,980 to Kelkar et al. a process is described for making novel catalysts, catalyst supports, and absorbers comprising synthetic hydrotalcite-like binders. The synthesis of the typical sheet hydrotalcite involves reacting pseudoboehmite to which acetic acid has been added to peptize the pseudo-boehmite. This is then mixed with magnesia. More importantly, the patent summary states clearly that the invention uses mono carboxylic organic acids such as formic, propionic and isobutyric acid. In this patent the conventional approaches to preparing hydrotalcites are presented.
In U.S. Pat. No. 6,539,861 a process is disclosed for preparing a catalysts for synthesis gas production based on hydrotalcites. The method of preparation is again based, on the co-precipitation of soluble salts by mixing with base, for example, by the addition of a solution of RhCl3, Mg(NO3)2 and Al(NO3)3 to a solution of Na2CO3 and NaOH.
Also in U.S. Pat. No. 5,399,537 to Bhattacharyya in the preparation of nickel-containing catalysts based on hydrotalcite use is made of the co-precipitation of soluble magnesium and aluminum salts.
In U.S. Pat No. 5,591,418 to Bhattacharyya a catalyst for removing sulfur oxides or nitrogen oxides from a gaseous mixture is made by calcining an anionic clay, said anionic clay having been prepared by co-precipitation of a solution of Mg(NO3)2, Al(NO3)3 and Ce(NO3)3. The product again is filtered and repeatedly washed with de-ionized water.
In U.S. Pat. No. 5,114,898/WO9110505 Pinnavaia et al. describe layered double hydroxide sorbents for the removal of sulfur oxide(s) from flue gases, which layered double hydroxide is prepared by reacting a solution of Al and Mg nitrates or chlorides with a solution of NAOH and Na2CO3. In U.S. Pat. No. 5,079,203/WO 9118670 layered double hydroxides intercalated with polyoxo anions are described, with the parent clay being made by co-precipitation techniques.
In U.S. Pat. No. 5,578,286, assigned to Alcoa, a process for the preparation of meixnerite is described. Said meixnerite may be contacted with a dicarboxylate or polycarboxylate anion to form a hydrotalcite-like material. In examples 5-6 hydromagnesite is contacted with pseudoboehmite under high CO2 pressure to obtain hydrotalcite.
In U.S. Pat. No. 4,946,581 and U.S. Pat. No. 4,952,382 to van Broekhoven co-precipitation of soluble salts such as Mg(NO3)2 and Al(NO3)3 with, and without the incorporation of rare earth salts was used for the preparation of anionic clays as catalyst components and additives. A variety of anions and di- and tri-valent cations are described.
In U.S. Pat. No. 5,814,291 (Kelkar ""291) discloses contacting an aluminum source with a magnesium source and an inorganic acid to make hydrotalcite-like material.
As indicated in the description of the prior art given-above, there are many applications of anionic clays. These include but are not restricted to: catalysts, adsorbents, drilling muds, catalyst supports and carriers, extenders and applications in the medical field. In particular van Broekhoven has described their use in SOx abatement chemistry.
Because of this wide variety of large-scale commercial applications for these materials, new processes utilizing alternative inexpensive raw materials are needed to provide a more cost-effective and environmentally compatible processes for making anionic clays. In particular, from the prior art described above one can conclude that the preparation process can be improved in the following ways: the use of cheaper sources of reactants, processes for easier handling of the reactants, so that there is no need for washing or filtration, eliminating the filtration problems associated with these fine-particled materials, the avoidance of alkali metals (which can be particularly disadvantageous for certain catalytic applications): In prior art preparations, organic acids were used to peptize alumina. The use of organic acids is expensive and introduces an additional step in the synthesis process and is therefore not cost-effective. Further, in drying or calcining the anionic clay prepared by prior art processes gaseous emissions of nitrogen oxides, halogens, sulfur oxides, etc. are encountered which cause environmental pollution problems.
It is particularly important that in the production of divalent metal-Al-containing anionic clays in a process where boehmite is peptized in an aqueous slurry with an inorganic acid and a divalent metal source is added to the slurry in excess of the stoicheometric requirement, that there not be an undesireable level of divalent metal salts in the clay product that would create a need for additional washing and filtering.
The term xe2x80x9cdivalent metalxe2x80x9d will hereinafter be designated as xe2x80x9cMxe2x80x9d.
In one embodiment, the present invention is a process for preparing Mxe2x80x94Al-containing anionic clays. The process comprises first peptizing boehmite in a aqueous slurry with an acid. To that slurry is added an M source in an amount that is at least stoichiometric with respect to the aluminum in the boehmite. A base is then added to the slurry in an amount comprising from about 0.10 to about 0.30 moles of hydroxyl ions or equivalent thereof to moles of alumina (based on Al2O3) in the slurry, or at least the amount of base required to obtain a pH above 7 in the slurry, whichever is greater. The base may be added before, during or after the M source is added. The slurry is then aged.
Other embodiments of the invention encompass compositions made by the above processes and details about such processes including reactants and conditions.